Cyber-defense and cyber-resilience techniques sometimes fail in defeating cyber-attacks. One of the primary causes is the ineffectiveness of business process impact assessment in the enterprise network. In this paper, we propose a new business process impact assessment method, which measures the impact of an attack towards a business-process-support enterprise network and produces a numerical score for this impact. The key idea is that all attacks are performed by exploiting vulnerabilities in the enterprise network. So the impact scores for business processes are the function result of the severity of the vulnerabilities and the relations between vulnerabilities and business processes. This paper conducts a case study systematically and the result shows the effectiveness of our method.